The invention relates to simplified resistive touch screen systems including a resistive touch screen and an ADC (analog-to-digital converter) for digitizing x and y coordinates of touch points applied to the resistive touch screen, and to switch circuitry that energizes such resistive touch screens; the invention relates more particularly to circuitry and a method for avoiding the effects of variations, especially thermal drift of the resistances of switches in the switching circuitry, on the calibration of the ADC to the touch screen.
U.S. Pat. No. 5,717,321 (Kerth et al.) issued Feb. 10, 1998, is believed to constitute the closest prior art. The Kerth et al. patent acknowledges that there are two conventional techniques for energizing a resistive touch screen assembly (hereinafter "touch screen"), either by connecting a voltage source across the resistive touch screen or by forcing a current through it. The Kerth et al. patent teaches (1) that applying a constant voltage across the resistive touch screen wastes power because typically there is a wide range in the resistivity of the resistor layers of a resistive touch screen; (2) that a class A driver is used which must be biased for the lowest acceptable resistance of the touch screen; and (3) that the resulting excess bias current above that required for touch screens that have higher resistance is wasted.
To overcome these problems, Kerth et al. teach that it is desirable to use feedback from the output of an ADC connected to the touch screen to control a current DAC that supplies current to energize the touch screen to cause a voltage across it to equal a desired reference voltage.
Referring to FIG. 6 of U.S. Pat. No. 5,717,321 by Kerth et al., if the stylus 71 is pressed on the upper surface of touch screen 70, that brings the resulting "touch point" of a conductive sheet in contact with a resistive layer. The voltage of that touch point, and hence the voltage of the conductive sheet, represents the x or the y coordinate of the tip of the stylus. Thus, accurate position information for the point of contact can be obtained by measuring resistive voltage division levels along an x axis of an x resistive sheet and a y axis of a y resistive sheet, respectively, and converting such analog voltages to digital numbers which then represent the x and y coordinates of the present touch point (at which the tip of the stylus is pressed on the touch screen).
If the values of touch screen resistance, contact resistance, and/or switch resistance change, for example with respect to temperature, age, etc., then the resistive divider output voltage produced at the touch point caused by the current I.sub.T also changes. Therefore, the touch screen becomes "uncalibrated" relative to the analog-to-digital converter 78, which converts the analog voltage at the touch point to a digital number D.sub.OUT.
The circuit shown in FIG. 6 of U.S. Pat. No. 5,717,321 attempts to deal with this problem by providing a switch 76 that connects the -REF terminal of analog-to-digital converter 78 to ground. A switch 73 also is provided to connect the +V.sub.REF terminal of analog-to-digital converter 78 to the output of current DAC 72. Current DAC 72 forces a current I.sub.S through the "y" axis resistance of resistive sheet 74 of touch screen assembly 70. Switches 80 and 79 likewise "energize" resistive sheet 75 of touch screen 70. A complex feedback circuit controls the current output by current DAC 72. Note that the voltage produced at the output of current DAC 72 is a "floating" voltage that is not nearly as stable as the voltage that would be produced by a typical voltage reference circuit or a typical power supply. The Kerth et al. feedback arrangement substantially eliminates the effect of variation of the switch resistances of switches 73 and 76 (that are used to energize "y" resistive sheet 74 of touch screen 70) or 79 and 80 (that are used to energize "x" resistive sheet 75) on the full scale digital output number produced by analog-to-digital output 78. The feedback also eliminates the effects of variations in screen resistance and contact resistance on the full scale digital output number.
However, the feedback arrangement of Kerth et al. fails to provide calibration or compensation of resistivity variations across the resistive sheets 74 and 75 that result in accurate midpoint output voltages thereof. The Kerth et al. feedback arrangement also fails to provide any compensation of digitizing errors due to thermal drift in the current DAC 72.